This application claims the priority of Japanese Application No. 8-283672, filed Oct. 25, 1996, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a catalyst for removing nitrogen oxide contained in an exhaust gas from an internal combustion engine such as a vehicle engine, and to a method of efficiently removing nitrogen oxide from the exhaust gas.
The catalyst in accordance with the present invention has a high performance of removing nitrogen oxide from an exhaust gas containing oxygen burned under a lean condition as well as an exhaust gas burned under a condition of stoichiometric air-to-fuel ratio (A/F=about 14.7), has heat resistance, SO.sub.x poison resistance, and is suitable for cleaning an exhaust gas exhausted from a lean-burn engine.
In the recent trend of natural resource saving and environmental protection, there is a social need to operate a gasoline engine for a vehicle under lean-burn conditions. In connection with this trend, there has been progress to develop a catalyst (lean NO.sub.x catalyst) which can effectively remove nitrogen oxide (NO.sub.x) in an exhaust gas containing oxygen from a lean-burn engine.
In regard to exhaust gas cleaning catalysts for lean-burn engines, there is reported a catalyst supporting an alkali metal oxide and platinum on a porous supporter, for example, such catalyst is shown in Japanese Patent Application Laid-Open No. 6-31139.
Furthermore in Japanese Patent Application Laid-Open No. 8-24643, a catalyst supporting at least one kind of palladium, rhodium and platinum, and at least one of alkali metal, alkali earth metal and rare earth metal, on the porous supporter. Here, as an embodiment of the catalyst material, one catalyst contains at least one kind of palladium, rhodium and platinum, and cerium and barium, and another catalyst contains them and further supporting La, Zr, Li, K, Mg, etc. thereon.